1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a wide angle zoom lens system.
2. Description of the Prior Art
As a zoom lens system covering a wide angle of view, there is known a zoom lens system which comprises two forward and rearward lens groups, namely, a diverging lens group and a converging lens group, but such zoom lens system suffers from various difficulties still left to be generally overcome, such as the destruction of the balance of spherical aberration and coma attributable to the variation in focal length, the pronounced negative distortion at the short focal length side, the variation in distortion during the focal length condition at the opposite ends of zooming, the imbalance of the chromatic aberration of magnification attributable to the angle of view, the mismatching of the curvature of image plane attributable to chromatic difference, the difficulty encountered in reducing the size of the forward lens diameter to make the entire system more compact.
These difficulties come from the fact that the forward group is a diverging lens system, and some attempts to overcome these disadvantages have heretofore been made by the use of various diverging lens systems.
For example, as far as the distortion is concerned, success has been attained in reducing the distortion by using a positive meniscus lens, convex to the object side, as a first component forming the diverging lens system, but since the first component is a positive lens, oblique rays are intensely refracted by the first component and the rays emergent therefrom form greater angles with the optical axis and therefore, if the diaphragm position is regarded as being fixed, a greater angle of view requires the oblique rays entering the first component to pass through positions more distant from the optical axis and this has prevented the forward lens diameter from being sufficiently decreased.
There is also a zoom lens system in which the first component closest to the object side is comprised of a negative meniscus lens to thereby reduce the angles of deviation of oblique rays and correct the distortion, and this is considerably effective to achieve the purpose of reducing the forward lens diameter, but the residual distortion in such system exceeds -4% at the short focal length side having a half angle of view of the order of 31.degree., and this could not be sufficiently practical and has been considerably inferior to the fixed focus lens having a single focal length.